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Lecture 14

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University of Toronto St. George
John Stinchcombe

BIO220: Lecture 14 Part 1 Genetic Diversity in Agricultural Systems  The world has 50,000 edible plants and 60% of food energy that humans use only comes from 3 species: rice, maize, wheat. And only few genotypes are grown/harvested. We know something about how agriculture wheat, rice, and we know their ancestors and we know that they’re very diverse.  Vavilov: Center of Origins  Crops originated in areas where the diversity of their wild relatives is greatest. A lot of this was driven by crops that was domesticated and developed in the ‘Fertile Crescent’ (ex Barley, wheat= beer, grapes= wine) o Problem: Vavilov was Russian at the time, and the main competing plant biologist was Lysenko.  Lysenko  Discovered ‘vernalization’  early flowering in biennial crops could be produced or stimulated by cold treating them. SO many plants that have to go through winter has mechanisms that sense when winter has passed. So by cold treating the plants, it accelerates their life cycle, tricking that winter has passed. o Caused by epigenetic modification of flowering time genes, allowing the plant to ‘remember’ winter o Shows how the environment directly affects the behaviour of plants  useful for agriculture o Problem: This is a single generation process. So vernalization occurs, and all of the epigenetic modifications that occurred are reset at meiosis. So impossible for this vernalized state to be transferred across generations  So you can use vernalization as a one time shot, but can’t use it to breed earlier/later flowering  Soviet Union rejected Mendelian genetics and Darwinisim in favour of Lamarckian approach o Lysenko promoted by communist party leadership and they adopted Lysenko-ism as the official doctrine instead of Mendelian genetics or Darwinian evolution o Wanted to train ‘Southern crops’ to grow in the North through Lamarckian breeding o Short Term Repercussions: ~3000 biologists/geneticists (who believed in Mendelian genetics and Darwinism) fired, sent to labour camps or executed  So Vavilov died in prison for being an evolutionary geneticist o Long-Term Repercussions: Generations of lost Russian genetics and evolutionary biology and prominent evolutionists refused to return to the USSR. Catastrophic crop failures and famines due to agriculture rejection of Mendel and Darwin  Why mentioning this?  ‘Responsibilities of human societies in a changing world’  part of bio220 course description  What happens during domestication of crops? **2 things will happen simultaneously: o Severe Bottleneck: only a tiny subset of individuals of the wild population are chosen to be cultivated o Strong artificial selection: Humans breed and retain the best performing crop plants **humans select very efficiently and quickly the phenotypes that are useful to them. Will lead to really strong natural selection on:  Germination timing, seed size, nutrition  Consequences of Domestication: o Reduced genetic variation ** Know difference between Heterozygosity (H) and Polymorphism (P) Basis of heterozygosity is genetic, but polymorphism has a genetic basis and also a strong environmental component. So Heterozygosity is having two different alleles for a gene (ex. you get blonde gene from mom and brown gene from dad) and polymorphisms are different mutated versions of the same genes that existed in a species (ex. all the hair colour genes in humans) Why do we care about genetic variation in crops? Summary: 1. Gives clues to past artificial selection (what traits our ancestors selected) 2. Future improvement of crops (possible to keep breeding better crops without genetic engineering? Is there enough genetic variation remaining either in crops or their wild relatives that we can keep improving them without using genetic engineering) 3. Pest and pathogen management (can we breed crops that are more efficient in avoiding loss due to pests 1. Clues to Past Artificial Selection: Example: Domestication of maize from teosinte  Compare genetic variation of modern day corn and its ancestor, teosinte  We see that vast majority of points fall below 1:1 line. The genetic variation that is left in maize is substantially reduced compared to what was present in wild ancestor.  This loss of variation (43%) means what? Can we distinguish which loci have lost variation because of artificial selection vs. because of the bottleneck?  Distinguishing bottlenecks from bottlenecks+ selection o Bottlenecks and selection reduce the effective population size (Ne). o Ne=- The size of an idealized population with the same properties with respect to genetic drift and allele frequencies as the observed population  Effective population size o Census size (N)= total number of adults in a population o Effective size (Ne)= number of adults that breed o Effective size usually << Census size (actual size) o Size of an ideal population (every adult reproduces) in which genetic drift equals the rate in the real population o Effective size is most important for evolutionary analysis  Why does N not equal Ne? o Because there is a variation in number of progeny among individuals (not everyone has the same number of children) o There is an unequal sex ratio o Overlapping generations (mating between offspring and parents) o Fluctuations in population size  Population bottlenecks The number of individuals making genetic contributions to the net generation is almost always the total census number of individuals  Ne<< N  This happens even in the absence of selection  Ne can be measured as a ‘species average’ across the genome and Ne can also be measured for each gene separately For the first (left) graph, there’s a population and then the bottleneck lasts a couple of generations and then you see the p
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